Printers may be classified as single-sheet printers or continuous-roll printers. Single sheet printers include drive and handling means to advance one sheet of paper at a time past a print head so that characters may be printed thereon. As each sheet is printed, it is ejected to be received by the user. Continuous-roll printers include a roll of paper instead of a supply of single sheets of paper. As the roll of paper is unrolled, the end of the paper is advanced past the print head by feed rollers or other drive mechanism for printing. After a printing job is completed, a blade or knife cuts the printed paper or the paper is detached manually using a tear bar. Common continuous-roll printers include thermal paper fax machines and retail checkout registers.
It is common to use a geared drive system in a continuous-roll printer with a stepper motor as a power source. Typically, a stepper motor will turn a fixed number of degrees in response to a pulse of electricity or a command from a controller. Gears are used to connect the stepper motor to the drive mechanism to ensure that a fixed rotation translates to a fixed advancement of the paper from the paper roll. It should be noted that the use of a stepper motor is not required, as other power sources may be used to control the rotation of the drive source and the feed rollers to accurately position the paper in relation to the print head for precise printing.
When the stepper motor turns in the forward direction in a continuous-roll printer, the paper is unwound from the paper roll and advanced past the print head. Turning the stepper motor in the reverse direction engages the knife or cutter blade to cut the printed paper from the roll. Using the same motor for feeding paper through the printer and cutting the printed paper is economical.
Continuous-roll printers are designed to only print in the forward direction. The paper is not retracted or wound back onto the paper roll during or after printing. With a direct gear system, reversing the stepper motor results in reverse feeding of the paper. Therefore the stepper motor, when turning in reverse, decouples from the paper drive system as it engages the cutter mechanism.
A wrap spring slip clutch, hereinafter referred to as a slip clutch, with an overrunning torque connects the gear drive system and the cutter blade. Slip clutches are used to transmit power in one direction of rotation only (called the "locking rotation") and include teeth, ratchet or spring mechanisms that lock a driven part to a driving part when the driven part is turned in the locking direction. When the rotation of the driving part is reversed, the mechanism releases, causing the driven part to stop turning while the driving part continues to turn, or "overrun" the driven part.
Some slip clutches are designed with an "overrunning torque" or a mechanism that will not automatically release during reverse rotation. A slip clutch with an overrunning torque will transmit torque from the driven part to the driving part even in the reverse direction, but will slip if the torque required to drive the driven part exceeds the overrunning torque.
As an example, consider a slip clutch with an overrunning torque of 1 inch-ounce. This slip clutch will lock if driven in its locking rotation, transmitting rotation of the driving part to the driven part without slippage. In the reverse rotation, the clutch will slip if the load on the driven part exceeds 1 inch-ounce. Causing the clutch to slip, however, requires an amount of torque equal to the overrunning torque as a friction loss. In other words, a drive motor generating 10 inch-ounces of torque in the reverse direction through a clutch that is slipping wastes 1 inch-ounce of torque that are required to cause the clutch to slip. The effective torque of the motor is thereby reduced to 9 inch-ounces.
The slip clutch is configured so that a reverse rotation of the stepper motor causes a locking, or forward rotation of the slip clutch. When the stepper motor and gear drive are driven in reverse, the slip clutch locks, engaging the cutter blade to slice off a piece of paper. Afterwards, the stepper motor resumes its forward rotation, causing the slip clutch to turn in reverse. The clutch, however, will not release until the torque required to continue turning the driven part exceeds the overrunning torque. Therefore, the cutter blade will be lifted, as slip clutches are designed to have an overrunning torque greater than the torque required to lift the cutting blade out of the paper path. The cutter blade continues to lift until it reaches a stop or limit mechanism, preventing further rotation, greatly increasing the torque required to lift the blade, and causing the slip clutch to release.
Even after the blade is lifted and the clutch released the stepper motor must continue to expend energy overcoming the overrunning torque so the blade will not fall back into the paper path. This results in friction loss, is a waste of energy, and increases the cost of the printer because a larger stepper motor must be specified than is required to drive paper through the paper path for printing. Additionally, it is rare that a slip clutch has a constant overrunning torque during its lifetime because environmental conditions, wear, and age modify the behavior of the clutch over time. If the overrunning torque becomes too high, paper will not feed properly because too much of the stepper motor's torque is wasted overcoming the friction generated by the overrunning torque. If the overrunning torque becomes too low, the cutter blade will not open or may slip back down into the paper path during printing.
What is needed, therefore, is a device to allow a cutter blade to engage upon reverse rotation of the stepper motor, to disengage upon the consequent forward rotation of the stepper motor, and to maintain its position out of the paper path during printing without adding the friction associated with an overrunning-style slip clutch to the system.